Scientists at the U.S. Geological Survey have linked an earthquake in Oklahoma in 2011 to the injection of oil-drilling wastewater underground. The latest research adds to a long list of studies that point to a possible relationship between fracking operations and unusual geological activity recorded in various regions across the United States.

The study, published in the journal Geology, said that the 5.7 magnitude earthquake that occurred near Prague, Okla., on Nov. 6, 2011, could also be the largest and most devastating ever linked to hydraulic fracturing. It destroyed 14 homes and injured two people and was felt all the way to Milwaukee, more than 800 miles from the site.

U.S. natural gas production generates huge amounts of wastewater, used for hydraulic fracturing to create cracks in rocks and release natural gas, and also in extraction of oil from oil wells. Either way, a significant proportion of that water has to be disposed of and one method of getting rid of it is to inject it back underground in deep wells.

However, many scientists have raised concerns that this process can trigger earthquakes. Geological activity has been detected lately in places previously believed to be calm but now hosting major oil and gas projects, such as Arkansas, Texas, Ohio and Colorado.

According to data from Geology, the number of earthquakes in the middle parts of the United States has increased by 11 times compared to the number recorded 30 years ago. As other studies have also pointed to a link with injecting wastewater underground, the National Academy of Sciences has called for further research into such seismic events.

Still, the research from U.S. Geological Survey noted that wastewater has been pumped underground for over 17 years without causing any trouble. Researchers speculated that as wastewater filled compartments that used to be full of oil the pressure that was needed to keep the fluid going down built up, triggering the earthquake, said Heather Savage, geophysicist at Columbia University's Lamont-Doherty Earth Observatory and co-author of the report.

Geoffrey Abers, who also took part in the research, said that even though the amount of water injected into the well was not big, it may have caused a series of tremors that culminated in the main shock. He noted that an unexpectedly large earthquake triggered by a relatively small injection suggested that the risk of inducing big earthquakes from even small injection activities was likely to be higher than previously thought.

So far, an official account of the sequence of events has not been released and wastewater is still being injected at the site. In a statement responding to the paper, Oklahoma Geological Survey seismologist Austin Holland stated that "these earthquakes could be naturally occurring" and further investigation into the causes of the earthquake was underway.